Opacifying super-particles

ABSTRACT

Microspheric opacifying agents are provided by admixing an aqueous, partially condensed, aldehyde condensation product with an oily material containing an emulsifying agent thereby forming a water-in-oil emulsion, admixing an amphiphilic acid catalyst with the emulsion and polymerizing the condensation product to form discrete, substantially spherical, opaque, solid particles. Additionally, the solid particles may be separated from the oily continuous phase and admixed with water for the further removal of oil material, and thereby causing the formation of spherical agglomerates or &#34;super particles&#34; having a substantially greater opacity than the aforesaid discrete particles. The super particles are hollow, substantially spherical particles having substantially spherical, discontinuous walls composed of agglomerated substantially spherical particles.

This is a division of application Ser. No. 508,322, filed Sept. 23,1974.

This invention relates to a process for the preparation of opaque,substantially spherical, microscopic opacifying particles, and to theparticles produced by such process. More specifically, this inventionrelates to the preparation of opaque, substantially spherical particlesformed of a formaldehyde condensation product and to sphericalagglomerates having an unusually high opacifying ability.

The preparation of microscopic particles formed from natural andsynthetic polymeric materials has been described, for example, in U.S.Pat. No. 3,585,149 and U.S. Pat. No. 3,669,899 to A. E. Vassiliades etal. The opacifying agents described in the Vassiliades et al patentscomprise discrete, air-containing microcapsules having substantiallycontinuous, solid walls, and an average particle diameter in the below 2micron range. Generally, the microcapsules described in the aforesaidpatents are formed by preparing an oil-in-water emulsion and coating theemulsion droplets with a film-forming material, which is subsequentlyhardened. The oily core material is expelled from the microcapsules toform air-containing microcapsular opacifying agents. The expulsion ofthe oily material from the microcapsules is accomplished be heating themicrocapsules, for example, after the capsules have been coated by meansof an aqueous dispersion onto a paper substrate. Alternatively, the oilycore material may be expelled by subjecting the capsules to aspray-drying operation under relatively high temperature conditions. Inany case, the expulsion of the oily material presented a number ofdifficulties. For example, one problem faced by the operators of such asystem is that of the care necessary to insure that all of the oil iscompletely expelled from the capsules. This was generally time consumingand required relatively severe temperature conditions. Additionally,there is the problem of oily solvent recovery, which is both anenvironmental problem as well as an economic one. Thus, some of the oilysolvent materials employed are fairly toxic and could endanger thehealth of the operators of the paper machine dryers. Additionally, asubstantial portion of the oily material is lost to further use in thesystem using the evaporative recovery systems employed. Such losses, ofcourse, play a role in the economics and commercialization of suchsystems.

It has now been discovered that opaque, microscopic, pigment particlesmay be produced in a system which avoids many of the drawbacks of theaforesaid systems utilized in the production of microcapsular opacifyingagents. Thus, it has been discovered that opaque, substantiallyspherical particles may be produced by a process which comprisesadmixing an aqueous, partially condensed, aldehyde condensation productwith an oily material containing an emulsifying agent thereby forming awater-in-oil emulsion. Thereafter, an amphiphillic acid catalyst isadmixed with the emulsion and the condensation product is polymerized,thereby forming substantially spherical, solid, opaque polymericparticles

According to one aspect of the present invention, the polymerizedparticles are separated from the bulk of the oily material and admixedwith an aqueous liquid, preferably water, under increased temperatureconditions in order to remove the remainder of the oily material fromthe particles and form microscopic agglomerates of said substantiallyspherical particles, which agglomerates have an unexpectedly highopacifying power.

A further aspect of the present invention involves the formation ofagglomerates having substantially spherical walls, which walls areformed of substantially spherical, microparticles.

The process of the present invention avoids many of the drawbacks of theprevious systems wherein an oil-in-water emulsion is formed, since thesystem of the present invention permits the easy separation and recoveryof the oily material by simple centrifugation or distillation, ratherthan solvent evaporation. Thus, the oily material of the presentinvention may be easily and readily recycled back to the emulsificationoperation thus providing both safe handling and an economic system.

According to a preferred aspect of the present invention, themicroparticles are provided by a process which comprises forming aprepolymer of urea and formaldehyde and admixing an aqueous solution ofthe prepolymer with an oily material containing an emulsifying agent. Awater-in-oil emulsion is thereby formed and an amphiphilic acid catalystis admixed with the emulsion causing the prepolymer to polymerize. Theresultant particles are admixed with an aqueous liquid, such as water,under conditions of brisk agitation while heating the particles toremove residual oily material. Optionally, the particles may beseparated from the aqueous liquid.

An additional understanding of the invention will appear from thefollowing description and the drawings in which:

FIG. 1 is a flow sheet outlining a general procedure for carrying outthe invention; and

FIG. 2 is a surface view; and

FIG. 3 is a sectional view, both enlarged, illustrating the physicalstructure of the agglomerated particles of the present invention.

Referring now to FIG. 1, a water-immiscible oily liquid is introduced bymeans of line 10 for admixture with recycled oily material and asurface-active emulsifying agent 12. Suitable water-immiscible oilymaterials for use in the present invention include, for example, anyorganic solvent capable of acting as the continuous phase of awater-in-oil emulsion. Suitable solvents include aliphatic and aromaticsolvents, such as petroleum ethers, naphthas, mineral spirits, toluene,xylene, turpentine or the like. Similarly, ketones, esters, halogenatedhydrocarbons etc., may be suitably utilized in the process of thepresent invention. The preferred solvents are those having a relativelylow cost and a low toxicity, such as mineral spirits or xylene.

The emulsifying agent is admixed with oily material in amountssufficient to provide, for example, between about 0.005 and about 0.2part by weight of emulsifying agent per part of oily material,preferably between about 0.02 and about 0.08 part per part of oilymaterial.

Suitable surface-active emulsifying agents are those capable ofpromoting the formation of a water-in-oil emulsion. Such materialsinclude, for example, lanolin, lanolin derivatives, sorbitan monooleate,polyol oleates, ethylene oxide adducts of fatty acids, fatty alcohols,fatty amines and fatty amides, cholesterol derivatives, fatty aciddiethanol amides, ethylene oxide-propylene oxide block copolymericcondensation products and the like, such surface-active agents beingwell known in the art. The preferred emulsifying agents are the ethyleneoxide-propylene oxide block copolymeric condensation productscommercially available from BASF-Wyandotte Corporation under the names"Pluronic" and "Tetronic."

Meanwhile, a urea-formaldehyde prepolymer is provided by introducingurean and formaldehyde by means of line 14 into reactor 16 at a moleratio of formaldehyde to urea in the range of between about 1:1 to2.5:1, preferably between about 1.2:1 and 1.5:1. The reaction takesplace in an aqueous solution at about 50% solids, at a pH of 9-10, and atemperature between about 50 and 120° C. The reaction time is controlledso as to produce a substantially clear prepolymer solution when thereaction mixture is cooled to room temperature. Such reactions are wellknown in the art. Although the foregoing discussion has been directedtowards a urea-formaldehyde prepolymer, any suitable partially condensedaldehyde condensation product may be employed for the formation of aninternal phase of the emulsion. Accordingly, for example, anycarbamide-aldehyde condensation product that is compatible with aqueoussolution polymerization is suitable for use in the present invention.Accordingly, other acid or base-catalyzed co-reactants may be employedincluding condensation reaction products of formaldehyde with phenols,such as, hydroxybenzene (phenol), m-cresol and 3, 5-xylenol carbamides,such as, urea; triazines, such as, melamine; amino and amido compounds,such as aniline, p-toluenesulfonamide, ethyleneurea and guanidine;ketones, such as, acetone and cyclohexanone, or combinations of thesematerials, with the provision that the prepolymer be insoluble in thewater-immiscible phase. Additionally, the prepolymer may be provided inany suitable aqueous medium including water, glycerol, poly (ethyleneoxide), glycols, or the like, may be suitably employed. Ureaformaldehydeis the preferred prepolymer. However, the substitution of melamine for 1to 2% by weight of urea provides a prepolymer will better storagestability. The expression "prepolymer" as utilized herein is intended tomean the initial, water soluble, reaction product of the carbamide andthe aldehyde. In the case of urea and formaldehyde, the prepolymerincludes methylol ureas and the oligomers of methylol ureas.

The prepolymer in the aqueous medium is withdrawn form the reactor 16 bymeans of line 18 and admixed with the water-immisciblesolvent-emulsifying agent mixture in line 20, and the resultingadmixture is passed by means of line 22 into mixer 24 wherein awater-in-oil emulsion is formed under conditions of brisk agitation. Thewater-in-oil emulsion may be prepared batch-wise, utilizing a tank witha high shear agitation, or continuously by combining the oil and waterphases into an in-line mixer, e.g., a Homomixer or Sonulator.Preferably, the agitation is conducted in a manner such that theemulsion droplets have an average particle diameter below about 5microns, preferably in the range of between about 0.5 and about 2microns. Alternatively, a suitable inorganic opacifying pigment, such asTiO₂, Al₂ O₃, barytes (BaSO₄), clay, ZnO, Ca(SO₄)², talc, and the likemay be provided in the emulsion. Preferred inorganic pigments for thepurpose of the present invention are TiO₂, Al₂ O₃, BaSO₄, clay and ZnO,with TiO₂ being especially preferred.

The addition of the inorganic pigment particles by means of line 26results in the incorporation of the pigment into the ultimate structureof the opacifying particles of the present invention. Alternatively, theinorganic pigment may be added to the emulsion by means of line 27 whereit migrates to the oil/water interface. When the particles of thepresent invention are ultimately formed, as thereinafter described, theinorganic opacifying pigment becomes incorporated in the polymericstructure at a point depending upon its position during thepolymerization step, e.g, at the particle-solvent interface orhomogeneously distributed throughout the polymer phase.

Although it is possible to incorporate an inorganic pigment particlesinto the structure of the polymeric particles of the present invention,highly opaque particles may be provided in the absence of such inorganicpigment particles. Accordingly, since the resulting polymeric pigment isopaque and not transparent, the practice of the present invention may beconducted without the use of the aforesaid inorganic opacifyingpigments.

The ratio of the internal, aqueous phase, to the external,water-immiscible solvent phase, is preferably in the range of betweenabout 0.4 and about 3 parts by weight, preferably between about 1 to 2parts by weight of the internal phase per part by weight of the externalphase. Although it is possible to utilize a higher ratio of internal toexternal phase, it is preferred to use an approximately 2 to 1 ratio.The resulting emulsion that is withdrawn from the mixer 24 by means ofline 28 have a viscosity in the range of between about 2 and 2000centipoises. Preferably, the viscosity of the resulting emulsion is lowand water-like.

Next, the emulsion is introduced to the polymerization reactor 30 alongwith an amphiphilic, acidic polyermization catalyst, having anionization constant greater than about 10⁻⁴, which is introduced bymeans of line 32. Suitable polymerization catalysts for the purpose ofthe present invention include, for example, polymerization catalyststhat are soluble in the continuous oily phase, but which have asignificant affinity for the internal, or water phase, such as anhydroushydrochloric acid, SO₂, SO₃, BF₃, BF₃ etherate, titanium tetrachloride,phosphoric acid, phosphorous pentachloride, silicontetrachloride,phosphorous trichloride, sulfuryl chloride, and the like; organiccarboxylic acids such as formic acid, acetic acid, trichloroacetic acid,and the like; alkyl acid phosphates, such as monoethyl acid phosphate,monoamyl acid phosphate, monobutyl acid phosphate, diethyl acidphosphate, and the like; substituted sulfamic acids, etc. Preferably,the acid catalyst is employed in amounts necessary to bring the final pHof the prepolymer phase to a pH of between about 0.5 and about 4,preferably between about 1 and about 2.

The catalyst is added at about ambient temperatures under mildagitation, preferably in the temperature range of about 10° to about 25°C. The polymerization reaction is exothermic, resulting in a temperaturerise, so that the polymerization reaction occurs at temperatures in therange of between about 30° and about 70° C, preferably in the range ofbetween about 40° and about 50° C. Since the acid-catalyzed condensationpolymerization is exothermic, cooling means (not shown) must be utilizedin connection with reactor 30 in order to keep the polymerizationtemperatures in the preferred range. Accordingly, costly heating meansfor reactor 30 are not needed. The polymerization reaction is conductedfor between about 0.25 and about 4 hours, preferably between about 0.5and about 2 hours.

As employed herein, the term "amphiphilic catalyst" means that thecatalyst possesses at least some significant affinity for both theaqueous and the oily phase of the emulsion, and thus is neithercompletely hydrophilic nor completely lipophilic. The employment of anamphiphilic polymerization catalyst permits the addition of saidcatalyst after the formation of the emulsion, since it is capable ofpassing through the continuous solvent phase to the water-oil interface,effectively catalyzing the polymerization of the urea-formaldehyde.Thus, the use of such catalysts permits the control of theemulsification operation including the emulsion droplet side withoutconcern of any premature gelling, which might occur if a catalyst wereintroduced into the prepolymer solution before emulsification. Anotheradvantage of using such a catalyst is that it will not prematurelyprecipate any added inorganic opacifying pigments, such as titaniumdioxide, out of the suspension in the prepolymer solution, which couldoccur if a water-soluble acid polymerization catalyst were added alongwith the aqueous, internal phase prior to the formation of the emulsion.

A dispersion of the polymerized urea-formaldehyde particles and waterdroplets in the water-immiscible oily solvent, i.e., a "solventdispersion," is withdrawn from reactor 30 by means of line 34 and passedto a vessel 36 by means of valve 35 and line 37. In vessel 36 thesolvent dispersion is separated into a substantially clear supernatentphase comprising the oily solvent, e.g., xylene, and most of theemulsifying agent. This phase is removed from vessel 36 by means of theline 40. The remaining phase is a heavy phase which comprises residualsolvent and the solid polymer particles. The heavy phase is termed an"inverted sludge" phase, since the system fed to the vessel 36 had beenin the form of a water-in-oil emulsion, and the system has now invertedto an oil-in-water emulsion.

The phase separation in vessel 36 may be accomplished by various meansincluding heating the solvent dispersion to a temperature, for example,in the range of between about 35° and about 70° C, preferably betweenabout 40° and about 50° C. Alternatively, the solvent dispersion may besubjected to direct centrifugation. Still another means for effectingthe phase separation is by diluting the solvent dispersion by additingof solvent, e.g., xylene, by means of process line 38. The phaseseparation can also be accomplished by subjecting the solvent dispersionto high shear mixing. In any event, after treatment, the solventdispersion is subjected to settling and decantation or centrifugation(by conventional means not shown). Regardless of the phase separationmeans employed, it is important to retain at least a small amount of thesolvent, e.g., xylene, in the inverted sludge. Suitable amounts includebetween about 0.2 and about 2, perferably between about 0.5 and about 1parts by weight solvent per part of polymer solids.

As previously indicated, the supernatent liquid comprising the solventand most of the emulsifying agent is withdrawn from the separator 36 bymeans of line 40 and the solvent is passed to a recovery system 42 whichincludes a liquid-liquid separator wherein the organic solvent isseparated from residual water and catalyst and recycled by means of line44 for admixture with a solvent make-up present in line 10. Thus, inthis manner, the solvent or external phase may be easily recoveredwithout the use of exotic recovery equipment normally associated withthe collection of volatilized solvents, and may be easily recycled forreuse in the process. Water is withdrawn from recovery system 42 bymeans of line 46 and may be subjected to waste treatment for recycle ofthe water for use in the process, or alternatively, the water may bepassed to disposal.

The resultant inverted sludge containing solid polymerized particles iswithdrawn from the separator 36 by means of line 48, and the particlesmay be passed by means of valve 50 and line 52 to drier 54 whereinresidual solvent is removed and the particles are obtained in line 56 ina dry randomly agglomerated form. If the acidic catalyst present in theinverted sludge in line 52 is neutralized by addition of a base, such asan alkali metal hydroxide, such as sodium hydroxide, introduced by meansof line 53, the resultant dried particles are substantially discretewhen removed from drier 54. The resultant particles have an averageparticle size of below about 2 microns, preferably between about 0.5 andabout 1.0 microns and may be employed as opacifying pigments, to providea relatively high opacity in the form of an opaque coating which iswhite, in the case of urea-formaldehyde.

According the another and more preferred aspect of the presentinvention, the solid polymeric particles present in the inverted sludgeare withdrawn from separator 36 by means of line 48, three-way valve 50and, line 58 and are passed to solvent removal tank 60, wherein anaqueous liquid such as water is introduced by means of line 62 fordiluting the inverted sludge and further removal of the water-immiscibleorganic solvent. The polymeric particles in vessel 60 are subjected toagitation and heating either directly employing live steam or indirectlyusing conventional heating means to a temperature in the range ofbetween about 120° and about 250° F, preferably between about 190° andabout 212° F for the removal of residual solvent by steam distillation,and to provide additional acid-catalyzed curing of the polymericparticles. Surprisingly, under the influence of shear, acid and heatredispersion of the polymeric particles in the aqueous medium results inthe formation of substantially spherically agglomerated opacifyingparticles wherein the walls of the particles are themselves formed ofmicropheres. Temperatures, for example, in the range of between about120° and about 250° F are utilized to "set" the spherical agglomeratestructure. Once the formation of spherically agglomerated opacifyingparticles has been achieved, residual oily solvent can be removed atlower temperatures under reduced pressure is desired, with no impairmentof the opacifying power of the final product. The resulting agglomeratedparticles are illustrated in FIG. 2 of the drawings.

Alternatively, the solvent dispersion from the Reactor 30 can bedischarged through line 34 through valve 35 and line 39 to line 58 andinto vessel 60 where water is added (line 62) and the mixture agitatedunder high shear to form an oil-in-water emulsion. The oily solvent isthen removed by steam distillation, as described in the preceedingparagraph, to produce the spherically agglomerated particles illustratedin FIG. 2.

FIG. 2 is a photomicrograph illustrating the nature of a preferredopacifying particle of the present invention which may be termed a"super-agglomerate," since it is composed of a shell formed ofagglomerated secondary particles, which are substantially spherical andsubstantially solid throughout, and which are themselves formed ofclusters of substantially spherical primary particles. As seen in FIG.2, the super-agglomerate is hollow on the inside of the outer shell.Although it is not intended to limit the invention to any particulartheory, it is believed that the secondary particles are derived from thepolymerization of an aqueous prepolymer droplet. Thus, the size of thesecondary particles is controlled by the droplet size of the initialwater-in-oil emulsion droplets formed during the initial emulsificationstep. The aqueous redispersion system in vessel 60 involves anoil-in-water emulsion with the secondary particles concentrated at thesolvent-water interface. The subsequent heating of this material duringthe initial phase of the distillation step results in the post-curingand fusion of the particles into a rigid substantially sphericalstructure.

The secondary particles are irregular and bumpy and are composed ofsmaller primary particles. Although the primary particles have beendescribed as substantially spherical, with increasedformaldehyde-to-urea ratio this differentiation becomes less distinct,so that the secondary particles have a smoother, only slightly pebbledsurface. FIG. 3 is a sectional photomicrograph illustrating the interiorof a super-agglomerate and demonstrates that the particles are hollow.

The super-agglomerate particles have an opacity which provides a two tothree-fold increase of that of the discrete, substantially sphericalsecondary particles that are withdrawn by means of line 52, whichparticles have not been subjected to heating under shear. Thesuperagglomerates have an average particle diameter of between about 1and about 20 microns, preferably between about 2 and about 7 microns.

Solvent vapor is steam distilled from the solvent removal vessel 60 inthe form of a mixture of residual xylene with water by means of line 63and passed through a condenser (not shown) and then on to join processline 40. Meanwhile, the aqueous dispersion of the substantiallyspherical agglomerates is withdrawn from vessel 60 by means of line 64and valve 66 and is withdrawn by means of line 68. The aqueous slurrythat is withdrawn by means of line 68 may be employed as a coating forthe direct application of the opacifying agents onto the desiredsubstrate, such as paper, with the incorporation of a suitable bindermaterial. The resulting substrate is then dried under conventional paperdrying conditions for removal of the moisture and the resulting coatedsubstrate has a high degree of opacity. The Kubelka-Munk lightscattering coefficient of such a coating formulated from 100 parts byweight of these pigment particles and 10 parts by weight of aconventional paper coating binder is between about 2,000 and about 6,000cm² per gram. Likewise, the resulting slurry may be incorporated in asurface finish, such as paint, to provide a high degree of opacitythereto.

Alternatively, the aqueous slurry withdrawn from vessel 60 by means ofline 64 may be passed by means of line 70 to a solid-liquid separator72, such as, for example, a centrifuge, for water removal by means ofline 74, and the resultant particles may be passed by means of line 76to dryer 54 in order to produce agglomerates in powder form. It has beenfound that the adjustment of the pH to a value greater than 8 prior todrying greatly facilitates the production of a free-flowing, non-cakingpowder during the drying process. The resultant polymeric opacifyingagents may be incorporated in paint or may be redispersed in a suitableaqueous or nonaqueous liquid with the addition of a binder and employedin the coating of paper or some other substrate, such as plastic, fabricor textile webs wherein it is desired to increase the opacity of suchsubstrate. If desired, the product from line 76 can be passed to line 77to wash tank 78 for washing to remove residual emulsifying agents byresuspending it in additional water in vessel 78. The washedsuper-particles are then passed by means of line 79 to separator 72.

These organic opacifying pigment particles in the form of the aqueousslurry from line 68, the wet cake from line 76, or the dry powder fromline 56 can also be added to paper furnish, that is, the slurry ofcellulose pulp fibers, sizing agents and other additives, and used toproduce paper by conventional papermaking techniques, providing a paperwith greatly increased opacity.

The following examples illustrate the production of the opacifyingpigments of the present invention and constitute the best modescontemplated for carrying out the present invention.

EXAMPLE 1

Ninety grams of urea are added to a solution of 165 grams of 37% aqueousformaldehyde and 45 grams of water, adjusted to pH 9.3 with NaOH andheated for 1 hour at 65° to yield a prepolymer solution containing about50% solids. Using a Waring Blendor, 140 grams of this prepolymersolution are emulsified in a solution of 6 grams of a polyethyleneoxide-polypropylene oxide block copolymer ("Pluronic L 122 fromBASF-Wyandotte Company) emulsifier dissolved in one-hundred grams oftoluene to produce a low-viscosity water-in-oil emulsion.

The emulsion then is treated with 4 milliliters of a 33% by weightsolution of titanium tetrachloride in toluene, resulting in anexothermic reaction which raises the temperature from about 28° to about50° C. After stirring for 2 hours, the resulting "solvent dispersion"consists of water droplets and solid polymer particles dispersed in theoil phase, with little or no tendency for coagulation of the polymerparticles. This is separated into a clear supernatant phase, containingthe oily solvent and some of the emulsifying agent, and a heavier phase,the "inverted Sludge", containing about 40% solids and about 20% oilysolvent. The phase separation is accomplished by centrifugation. Theinverted sludge is redispersed in about 200 grams of water and issubjected to high-shear agitation while heating to steam distill off theoily solvent as a mixture with a portion of the excess water.

The resulting product is free of toluene and consists of an aqueousdispersion of 0.25 to 2μ polymer particles, which have fused togetherinto substantially spherical agglomerates (super-particles) 1 to 5μ indiameter. The super-particles are made basic with ammonia and blendedwith a carboxylated styrene-butadiene rubber (SBR) latex paper coatingadhesive (Dow 620 SBR latex, ten parts by weight latex solids toone-hundred parts polymer solids) and coated on a paper substrate.

The Kubelka-Munk scatter coefficient of the paper coating is measuredusing a Huygen model 2100 digital opacimeter and computational methodsdescribed in the literature. A scatter coefficient of approximately 4000cm² /gram is obtained. Formulated and coated under the same conditions,a water-dispersable paper-coating grade of anatase TiO₂ gives coatingswith a scatter coefficient of 3,800 to 4,000 cm² /gram.

EXAMPLE 2

Two hundred grams of a prepolymer solution prepared by heating 165 gramsof 37% aqueous formaldehyde, 45 grams of water, 89 grams of urea and 1gram of melamine, adjusted to pH 9.3 with sodium hydroxide, at 70° C forone hour are emulsified in a solution of six grams of a polyethyleneoxide-polypropylene oxide block copolymer attached to a centralamine-functional group (Tetronic 1502 from BASF-Wyandotte Company),which acts as an emulsifying agent, dissolved in one-hundred grams ofxylene. 0.06 milliequivalents of sulfur dioxide (as a 4 normal solutionin xylene) are added to the water-in-oil emulsion, resulting in atemperature rise from about 28° to about 48° C. After one hour theresulting solvent dispersion is centrifuged, the inverted sludge phaseis mixed with water and steam distilled under high-shear agitation. Theproduct is free of residual xylene, and a coating on paper, prepared asdescribed in Example 1, has a scatter coefficient of about 4,500 cm²/gram.

EXAMPLE 3

A urea-formaldehyde-melamine prepolymer solution is prepared from 165grams of 37% aqueous formaldehyde, 45 grams of water, 89 grams of ureaand 1 gram of melamine, adjusted to pH 9.3 and heated for 1 hour at 65°C. One-hundred seventy grams of this prepolymer solution are emulsifiedin a solution of six grams of Tetronic 1502 dissolved in one-hundredgrams of xylene, and 0.06 milliequivalents of sulfur dioxide (about 4normal in xylene solution) is added, resulting in an exothermicreaction. After stirring for 1 hour, the solvent dispersion has littleor no precipitate. Without any preliminary phase separation, the productis mixed with water to give about 16% total solids, and this "aqueousredispersion" is steam distilled under high-shear agitation to yield aproduct free of of residual xylene, closely resembling that described inExample 1. A paper coating prepared as described in Example 1 has ascatter coefficient of about 4,000 cm² /gram.

The aqueous dispersion of spherical agglomerates is centrifuged, theaqueous supernatant phase is decanted, the precipitate is mixed withfresh water at about 10% solids and stirred for one hour. The washedproduct is concentrated by centrifugation and formulated into a papercoating as described in Example 1. A scatter coefficient of about 4,800cm² /gram is observed.

EXAMPLE 4

Eighty grams of a urea-formaldehyde prepolymer solution prepared asdescribed in Example 1 are emulsified in a mixed emulsifier solution offour grams of Pluronic L 122, 2 grams of sorbitan monooleate (Span 80from ICI America), 1 gram of Pluronic L 63 (a lower molecularpolyethylene oxidepolypropylene oxide than Pluronic L 122, containing ahigher percent polyethylene oxide) and one-hundred grams of xylene. Theresulting water-in-oil emulsion is treated with two milliliters of a 50%(by weight) solution of monobutyl acid phosphate in xylene and stirredfor 2 hours. The resulting solvent dispersion is centrifuged, and theheavier inverted sludge phase is dried in an oven at 80° C to obtain amaterial free of solvent. This is redispersed in water made basic withammonia and mixed with an SBR latex adhesive (ten parts latex solids toone-hundred parts polymeric opacifier solids) to give a coating whichexhibits a scatter coefficient of about 2,000 cm² /gram.

EXAMPLE 5

One-hundred grams of a urea-formaldehyde prepolymer solution, preparedas described in Example 2, are emulsified in a solution of two grams ofstearic acid diethanolamide (Schercomid ST from Scher Bros., Inc.)dissolved in one-hundred grams of xylene. Four milliliters of a 33% byweight solution of titanium tetrachloride in xylene are added, theexothermic reaction occurs and the dispersion is stirred for two hours.The products is centrifuged, and the heavier inverted sludge phase ismixed with about two-hundred grams of water and steam distilled underhigh-shear agitation. A coating prepared from the solvent-free productas described in Example 1 has a scatter coefficient of about 3,000 cm²/gram.

EXAMPLE 6

One-hundred-forty grams of a prepolymer solution prepared as describedin Example 2 are emulsified in a solution of six grams of tall oil fattyacid diethanolamide (Schercomid TO-1 from Scher Bros., Inc.), dissolvedin one-hundred grams of a chiefly aliphatic solvent (Shell-Sol 70 fromShell Oil Co., 98% saturates, 2% olefins and aromatics; initial boilingpoint 160° C, end point 180° C). Seven milliliters of about 4 normalsulfur dioxide in xylene solution are added to the water-in-oilemulsion, resulting in a temperature rise from about 30° to about 50° C.After stirring for about two hours, there is no significant precipitate.The emulsion is centrifuged for forty-five minutes, and the invertedsludge is redispersed in about two-hundred grams of water containing twograms of polyoxyethylene sorbitan monolaurate (Tween 20 from ICIAmerica). The organic solvent is distilled off under high-shearagitation to give polymeric opacifiers which are formulated into a papercoating as described in Example 1. This has a scatter coefficient ofabout 1,400 cm² /gram.

EXAMPLE 7

A urea-formaldehyde prepolymer solution is prepared by heating anaqueous solution of two-hundred-twenty grams of 37% formaldehyde and 82grams of urea, adjusted to pH 8 with triethanolamine, at 70° C for onehour. Fifty grams of this prepolymer solution are mixed with thirtygrams of a 50% by weight dispersion of titanium dioxide in distilledwater, and adjusted to pH 6 with dilute sulfuric acid. The aqueousdispersion is emulsified in a solution of six grams of Pluronic L122,three grams of sorbitan monooleate (commerically available as Span 80from ICI America) and 1.5 grams of apoly(oxyethylene)-poly(oxypropylene) block copolymer with 30%poly(oxyethylene) and a molecular weight of about 2500 (commerciallyavailable as Pluronic L 63) in one-hundred grams of xylene to yield awater-in-oil emulsion. This is treated with four milliliters of a 50% byweight solution of monobutyl acid phosphate in toluene and stirred fortwo hours.

The solids are separated from the continuous phase by centrifugation anddried at 80° C in a forced draft oven. The residue is redispersed inwater, yielding discrete, spherical particles averaging 2-3 microns indiameter, with titanium dioxide particles clearly visible inside eachsphere. No free titanium dioxide is discernable in the aqueous phase at1000X magnification.

The aqueous dispersion is formulated with a latex adhesive as describedin Example 1 and coated on paper to yield a scatter coefficient of about1700 cm² /gram. Electron micrographs of the dry product showapproximately spherical particles; X-ray analysis of single particles ona scanning electron microscope show high titanium loading.

EXAMPLE 8

A urea-formaldehyde prepolymer is prepared in a continuous manner byblending together a stream of 37% aqueous formaldehyde, adjusted to pH9.5 with sodium hydroxide and a stream of 67% aqueous urea, heated to65° C to prevent crystallization, in a ratio corresponding to aformaldehyde to urea ratio of 1.3 to 1, and then passing this mixturethrough a heated coil at a temperature of about 95° C, with a residencetime in the reactor of about 3 minutes. Upon leaving the heated reactorcoil the prepolymer solution is cooled in a heat exchanger and thenemulsified in a solution of six parts of Tetronic 1502 in one-hundredparts xylene, in a ratio of one-hundred forty parts of prepolymersolution to one-hundred parts of xylene. The resulting emulsion iscooled to about 20° C in a heat exchanger and then passed into a mixingzone where 0.06 milliequivalent of sulfur dioxide is added to about 250parts of the emulsion. The catalyzed emulsion passed through a tubereactor and, after a reactor residence time of about one-half hour, issteam distilled with excess water under high-shear. Coatings preparedfrom the product and from washed product as described in Examples 1 and3 have scatter coefficients of about 4000 and about 5000 cm² /gram,respectively.

EXAMPLE 9

Paper handsheets were prepared using microcapsular opacifiers preparedas described in Example 1 and anatase titanium dioxide as fillers, addedto the furnish to enhance opacity and brightness.

Three samples of a 300 gram "air dry" mixture of 50% pine and 50%hardwood pulps, each, were disintegrated in a dynapulper and wererefined in a valley beater to a Canadian Standard Freeness of 250-350.After each batch was pressed and shredded individually, the threebatches were combined and the composite was shredded until a 10.0 gramsample gave a Canadian Standard Freeness of 275-325. Moisture of thecomposite was obtained by dispersing several 10.0 gram samples in 100milliliters of distilled water under a Hamilton Beech dispersator for2-3 minutes, forming pads in a Buchner Funnel, drying the pads on a hotplate, and calculating the percentage moisture. This result was used tocalculate "bone dry" weights of fiber for paper furnish formulations.

Nine handsheets were prepared for each of the filler pigment samples(three at 5, 10, and 15% as bone dry weight, respectively) by weighingout 3.0 grams "bone dry" samples of fiber into plastic bottles, addingthe appropriate weight of filler (0.15, 0.30, and 0.45 grams "bone dry")as necessary, diluting with distilled water to 110 milliliters totalvolume, dispersing under the Hamilton Beech dispersator for 3 minutes,and forming in a Noble and Wood sheet mold, with the following results.

    ______________________________________                                                      G. E. Brightness                                                Description   F/W           Tappi Opacity                                     ______________________________________                                                      78.9/79.7     76.6                                               5% TiO.sub.2 81.4/82.2     81.1                                              10% TiO.sub.2 82.9/83.6     84.5                                              15% TiO.sub.2 84.1/85.4     87.1                                               5% microcap. opac.                                                                         81.5/82.2     80.5                                              10% microcap. opac.                                                                         83.7/85.2     84.6                                              15% microcap. opac.                                                                         84.5/86.1     86.6                                              ______________________________________                                    

The microcapsular opacifiers compared favorably with anatase titaniumdioxide in this application.

EXAMPLE 10

A sample of an aqueous dispersion of microcapsular opacifiers preparedas described in Example 2 (coating scatter coefficient about 4500 cm²/gram) is adjusted to pH 8.3 with sodium hydroxide and heated to drynessin an 80° C forced draft oven. Ten grams of the dry powder are dispersedin fifty grams of water, adjusted to pH 9 with ammonia and treated withtwo grams of Dow 620 latex (about 50% solids) to give a paper coatingwhich had a scatter coefficient of about 4700 cm² /gram.

What is claimed is:
 1. An opacified substrate, said substrate having asurface coating thereon, said coating consisting essentially ofopacifying agents, said opacifying agents consisting essentially ofsubstantially spherical superparticles, each of said super-particleshaving a substantially spherical, discontinuous shell composed ofagglomerated, discrete, organic, polymeric secondary particles, saidshell surrounding a substantially spherical hollow core, said secondaryparticles being substantially spherical and substantially solidthroughout.
 2. The substrate of claim 1, wherein said secondaryparticles are formed of urea-formaldehyde.
 3. The substrate of claim 1wherein said secondary particles contain inorganic pigment particles. 4.The substrate of claim 3 wherein said inorganic pigment particles areTiO₂.
 5. The substrate of claim 1, wherein said superparticles have anaverage diameter of between about 1 and about 20 microns.
 6. Thesubstrate of claim 5 wherein said superparticles have an averagediameter of between about 2 and about 7 microns.
 7. The substrate ofclaim 2 wherein said secondary particles contain TiO₂, saidsuper-particles having an average diameter of between about 2 and about7 microns.
 8. The substrate of claim 2 wherein said secondary particlesadditionally comprise melamine.